Heat shrinkable polymer films have gained substantial acceptance for use in meat packaging. This description will detail the usage of films for packaging meat, it being understood that these films are also suitable for packaging other products. The films embodying this invention are useful as heat shrinkable bags supplied to the meat packer with one open end, to be closed and sealed after insertion of the meat. After the product is inserted, air is normally evacuated, the open end of the bag is closed, such as by heat sealing, or applying a metal clip, and finally heat is applied, such as by hot water, to initiate film shrinkage about the meat.
In subsequent processing of the meat, the bag may be opened and the meat removed for further cutting of the meat into user cuts, for retail sale, for example, or for institutional use.
Successful shrink bags must satisfy a multiplicity of requirements imposed by both the bag producer and the bag user. Of primary importance to the bag user is the capability of the bag to survive physically intact the process of being filled, evacuated, sealed closed, and heat shrunk. The bag must also be strong enough to survive the meat handling involved in moving the contained meat, which may weigh 50 pounds or more, along the distribution system to the next processor, or to the user. Thus, the bag must physically protect the meat.
It is also highly desirable to the bag user that the bag serve as a barrier to infusion of gaseous materials from the surrounding environment. Of particular importance is provision of an effective barrier to infusion of oxygen, since oxygen is well known to cause spoilage of meat.
The bag producer requires a product which can be produced competitively while meeting the performance requirements of the user. Thus the bag material should be readily extrudable, and susceptible to orientation, with sufficient leeway in process parameters as to allow for efficient film production. The process should also be susceptible to efficient extended production operations. In the orientation process, the film must be tough enough to withstand the stretching. The orientation temperature should be a temperature which is economically achieved by the producer, and which provides for use of economical shrink processes by the bag user.
Conventional shrink bags have generally been constructed with ethylene vinyl acetate copolymers (EVA). In some cases the bags contain a layer of a Saran copolymer to serve as an oxygen barrier. Ethylene vinyl alcohol copolymer (EVOH) has also been suggested as the barrier layer.
Notwithstanding the advantages, shrink-bag packaging of meat is not without its difficulties, many of which are attributable to limitations in the film. As will be appreciated, the processes of stretching the film, and later shrinking it, expose the film to rather severe conditions, due to the nature of the operations.
It is especially important to appreciate that the film is particularly vulnerable to failure at conditions of operation, due to the relatively high temperatures to which it is exposed in the orientation and shrinking process.
The film must be susceptible to orientation without distortion, or separation of the multiple layers which are normally present in films of this nature. The film must be strong enough, at the orientation temperature to withstand the stretching without creation of holes, tears, or non-uniform zones of stretching.
In the case of blown tubular film, the film must be capable of supporting the stretching bubble during the orientation process. Finally, each of the layers of the film should be susceptible to orientation without fracture, separation, or creation of holes in the layer.
In packaging use, the film must respond to heat rapidly enough for commercial practicability, and yet must not exhibit such a level of shrink energy as would cause the film to pull apart or delaminate during shrinkage, under its own internal forces.
In U.S. Pat. No. 4,457,960 oriented multiple layer polymeric films comprise a barrier layer having two opposing surfaces; a second layer adheres to one surface of the first layer, said second layer being 10% to 90% linear low density polyethylene and 90% to 10% ethylene vinyl acetate copolymer; and a third layer adheres to the other surface of the first layer, the composition of said third layer being an ethylene vinyl acetate or a blend of 10% to 90% linear low density polyethylene with 90% to 10% of ethylene vinyl acetate. The ""960 patent further discloses a five layer film structure having a barrier layer; a second and third layer having essentially the same composition and a fourth and fifth layer having essentially the same composition with at least one of said pairs having at least 50% of an ethylene vinyl acetate copolymer, the remainder being linear low density polyethylene and at least one of the pairs comprises at least 10% linear low density polyethylene, the remainder being ethylene vinyl acetate copolymer.
In U.S. Pat. No. 4,853,265 the heat-shrinkable, oriented, multi-layer packaging film has at least two layers comprising an ethylene vinyl acetate copolymer. One of the ethylene vinyl acetate copolymer layers has a melt index difference of about 0.3 dg/minute from the melt index of the other ethylene vinyl acetate copolymer layer.
In U.S. Pat. No. 4,894,107 a process for making multiple layer polymeric films is disclosed. The films have a layer of vinylidene chloride copolymer between at least two other layers which contain ethylene vinyl acetate and optionally, linear low density polyethylene.
In U.S. Pat. No. 5,030,511 films are made from vinylidene chloride copolymer compositions comprising about 0.01% to 6% by weight of a processing aid wherein said processing aid has a molecular weight of less than about 700. The films also have a first and second layer wherein each of the first and second layers comprises an ethylene vinyl acetate copolymer having a melt index greater than about 1.2.
In U.S. Pat. No. 5,538,770 the polymeric film comprises first and second surface layers wherein said first and second surface comprise an olefinic polymer or copolymer and a vinylidene chloride polymeric layer disposed between said first and second layers wherein said vinylidene chloride polymeric layer comprises 100 parts by weight of at least one vinylidene chloride copolymer; between 4 and 15 parts by weight of a plasticizer and between 4 and 15 parts by weight of an acrylate/styrene copolymer.
Despite the known technology surrounding heat-shrinkable films, there is still a need to improve the film in order to meet the demands of the meat packaging industry.
Thus, it is an object of the invention to provide improved film structures for use in shrink bags wherein the shrink bags are capable of withstanding production and shrink processes.
The present invention provides an oriented multiple layer polymeric film useful in the packaging of meat. The present invention also provides for a process for manufacturing the oriented multiple layer polymeric film of the present invention. The present invention still further provides for heat shrinkable bags which are made of the oriented multiple layer polymeric film of the invention and are useful in the packaging of meat.
In one embodiment of the present invention the oriented multiple layer polymeric film has a first barrier layer, the first layer having two opposing surfaces; a second layer is adhered to one of the surfaces of the first layer; a third layer is adhered to the other surface of the first layer, a fourth layer is adhered to one of said second or said third layer; and a fifth layer is adhered to the other of said second or third layer.
In this embodiment the barrier layer comprises a polyvinylidene chloride methyl acrylate blend, the second and third layers independently comprise a blend of two ethylene vinyl copolymers wherein the melt index of the blend for each of the second and third layers are the same; the fourth layer comprises a blend of two ethylene vinyl copolymers and processing additives wherein the melt index for the EVA blend of said fourth layer is the same as the melt index for said second and third layers; and the fifth layer comprises a blend of linear low density polyethylene (LLDPE) and low density polyethylene (LDPE).
A preferred embodiment of the present invention also has a first barrier layer with two opposing surfaces, said first barrier layer comprising a polyvinylidene chloride methyl acrylate blend. A second and third layer adheres to the surfaces of said barrier layer comprising a blend of two different ethylene vinyl acetate copolymers (EVA) wherein the melt index for the EVA blend of each second and third layer are the same; wherein for each second and third layer one of the EVA is present in a range of about 65%-85%, preferably about 74%, based on the total weight of the layer; the other EVA is present in a range of about 15%-35%, preferably about 26%, based on the total weight of the layer. A fourth layer adheres to one of said second or third layer comprising a blend of two different EVAs and processing additives wherein the melt index for the EVA blend of said fourth layer is the same as the melt index for said second and third layers, and wherein one of the EVAs of the blend of EVAs is present at about 65%-85%, preferably about 72.5%, and the other EVA is present at about 15-35%, preferably about 25% based on the total weight of the layer, and the processing additives are present at about 0.1-10% and preferably about 2.5%, based on the weight of the layer. A fifth layer adheres to the other of said second or third layer comprising a blend of LLDPE and LDPE wherein the LLDPE is present at about 80-100%, and preferably about 90%, and the LDPE is present at about 0-20%, and preferably about 10%, based on the total weight of the layer.
A still further embodiment of the present invention has a first barrier layer with two opposing surfaces comprising polyvinylidene chloride methyl acrylate blend. A second and third layer adheres to the surfaces of said barrier layer each second or third layer comprising a blend of two different EVAs wherein the melt index for the EVA blend of each second and third layers are the same with the understanding that one of said second or third layer may contain additional processing additives. A fourth layer adheres to the surface of said second or said third layer containing a blend of two different EVAs and processing additives wherein the melt index for the EVA blend of said fourth layer is the same as the melt index for said second and third layer. A fifth layer adheres to the layer of said second or said third layer containing only a blend of two EVA and wherein said fifth layer comprises a blend of LLDPE and LDPE.
A still further preferred embodiment of the present invention has a first barrier layer with two opposing surfaces comprising polyvinylidene chloride methyl acrylate blend; a second and third layer adhere to the surfaces of said barrier layer wherein one of said second or third layer comprises a blend of EVAs wherein one EVA is present at about 65-85%, preferably about 74%, and the other EVA is present at about 15-35%, preferably about 26%; the other of said second or third layer comprises a blend of two different EVAs, and processing additives and wherein one of the EVAs of the blend of EVAs is present at about 65%-85%, preferably about 72.5%, and the other EVA is present at about 15-35%, preferably about 25% based on the total weight of the layer, and the processing additives are present at about 0.1-10% and preferably about 2.5%, based on the weight of the layer; a fourth layer adheres to the surface of said second or third layer containing a blend of two different EVAs and processing additives wherein said layer comprises a blend of two different EVAs and processing additives wherein one EVA of the EVA blend is present at about 65-85%, preferably about 72.5%, and the other EVA is present at about 15-35%, preferably about 25%, and the processing additives are present at about 0.5-10%, preferably about 2.5%; a fifth layer adheres to the layer of said second or said third layer comprising a blend of two different EVAs wherein said fifth layer comprises a blend of LLDPE and LDPE wherein the LLDPE is present at about 80-100%, preferably about 90% and the LDPE is present at about 0-20%, preferably about 10% based on the total weight of the layer.
In all the multiple layer films of this invention, the barrier layer is preferably polyvinylidene chloride methyl acrylate blend. Other barrier resins such as polyvinyl chloride, ethylene vinyl alcohol copolymer would also be suitable in the practice of this invention.
The films of this invention may optionally be subject to irradiation after the completed fabrication of the multiple layer film structure but before or after orientation of said film structure.
Irradiation doses of from 2MR to about 10MR are used to irradiate the films of the present invention. A more preferred irradiation dose for the films of the invention is from about 4MR to 6MR.
A substantial end use of the invention is in heat sealable shrink bags for utilization particularly in packaging of meat. Bags made according to the invention find particular utility in forming packages which are subjected to low temperature shrinking processes.